The present invention relates to a circuit testing method and apparatus and particularly to such method and apparatus for providing electrical contact and measurement of circuit elements without the requirement of contacting those elements physically.
Present day electronic circuitry typically employs a multiplicity of subunits in the form of integrated circuits, circuit boards and hybrid units which are individually mass produced and which each contain their own complex conductive pattern. These circuit elements are separately tested for their operability and may require individual specialized test fixtures for making multiple connections with the device-under-test. Test fixtures represent an additional expense in regard to design, operation and maintenance.
Recent efforts have been directed to testing individual circuit devices without forming an actual physical contact therewith. Instead, an electron beam is used to contact or scan conductive portions of the circuit device. See, for example, U.S. Pat. Nos. 4,415,851 to Langner et al and 4,417,203 to Pfeiffer et al as well as IBM Technical Disclosure Bulletin, Vol. 24, No. 11A, April, 1982 entitled "Tri-Potential Method for Testing Electrical Opens and Shorts in Multi-Layer Ceramic Packaging Modules" by Chang et al. Testing procedures as hereinbefore described involve electron beam reading and writing operations taking place within the stable portion of the secondary emission characteristic for the material being scanned. A given contact point is charged to a given value and other contact points, which are supposed to be connected thereto, are read out to see if they are at the same potential level. If they are not, a proper connection is absent, while if contact is indicated to supposedly unconnected terminals, then an improper connection or short is indicated.
In the aforementioned prior art method, electron flood guns are utilized to bring all metal parts on a circuit device to a given voltage level, and then, with the flood guns turned off, a selected connection is brought to a different voltage level with a writing beam. Various other connections on the device are read, as indicated above, to determine their connection or lack of connection with the point which is written. Unfortunately, the measurement of extremely small voltages is required and therefore the voltages read are subject to error. Moreover, each writing and reading procedure requires the device terminals to be brought to an initial voltage level before a circuit point is written. In general, all other circuit points are then measured with respect to the written circuit point requiring a lengthy scanning and comparison procedure.